1. Field of the Invention
This invention relates to Ca.sup.++ electrode systems, and more particularly to an electrode system using a solid state electroactive membrane as Ca.sup.++ sensitive material.
2. Prior Art
The utility of organo-phosphorus compositions in Ca.sup.++ sensitive electrodes has long been recognized. Most prominent among such electrodes has been the Orion electrode which contains the calcium salt of didecylphosphoric acid (exchanger) dissolved in di-(n-octyl)phenylphosphonate (DOPP-n/mediator). This electrode was characterized by a response time in pure solution of about 30 seconds. Response time in impure solutions was extended as long as ten minutes.
Subsequent analysis of the Orion electrode had disclosed that at least five compounds were existent in the electroactive composition, prompting an elevation of the effects of improving purity of the exchanger-mediator combination. Griffiths, Moody and Thomas, J. Inorg. Nucl. Chem 34; p 3043 (1972). A similar analysis was conducted by a second research group (Ruzicka, Hansen and Tjell, Anal. Chem Acta 67, p 155, (1973); however, the results of both investigations demonstrated minimal improvement with use of purified components.
An attempt to improve the Orion electrode was also made by Cosgrove, et al (U.S. Pat. No. 3,729,401) utilizing the calcium salt of di-[p-(1,1,3,3,-tetramethybutyl)phenyl] phosphoric acid (herein referred to as t-HDOPP.) The compound, whose physical state was described as "normally" solid, was dissolved in an organic phase and used without mediators. No concern is reflected by this disclosure as to method of preparation or purity of the t-HDOPP constituent. The fact that the composition was described as "normally" solid suggests that extreme purity was not considered to be a critical factor.
In U.S. Pat. No. 3,932,233, Ruzicka incorporates t-HDOPP into a solid state electrode having di(alkyl-)arylphosphonate as the mediator, the combination being fixed in a polyvinylchloride matrix to form a Ca.sup.++ sensitive membrane. The amount of mediator used was 5 to 50 times the amount of exchanger, with a comparable large amount of polyvinyl chloride.
The use of this solid state, organo-phosphorus composition having predominant mediator/PVC concentration produced improvements in selectivity over Na.sup.+ of a factor of 10.sup.2 and an improved detection limit approximately 20 times lower than previous electrodes. It has been reported, however, that these electrodes have time responses of 5-15 minutes when one day old, time responses of one minute for up to seven weeks, and are non-functional thereafter. Christoffersen and Johansen, Anal Chem Acta 81, p 191 (1976).
Although the preparation procedures for a di(aryl)phosphate were outlined by Ruzicka (U.S. Pat. No. 3,932,233) the purification step did not suggest any special methods or need for extreme purification of the di(aryl)phosphate exchanger. The extraction procedure disclosed by Ruzicka clearly indicates that normal purity was intended. Neither was any experimental data recited to suggest the need for special attention to the purification step.
Still another attempt to improve the Orion electrode was made by H. M. Brown, J. P. Pemberton and J. D. Owen as disclosed in Anal Chem Acta 85 pp 261-276 (1976). This electrode used the same electroactive material (t-HDOPP) as used previously by Cosgrove and Mask, but used the mediator (DOPP-n) of Ruzicka et al and Ross and the PVC matrix of Griffiths, et al. Although a unique procedure of using a lower percent of PVC was discovered to yield the best results in a microelectrode, conventional purity for t-HDOPP was considered adequate. Nothing had been disclosed by previous investigators to indicate the need for special attention to exchanger purity. Previous analyses of improved purity had not rendered any substantial improvement and the low concentration of exchanger in mediator and PVC would seem to suggest that efforts to use a highly purified exchanger would have little effect in view of the dilution of the t-HDOPP in a predominant mediator/PVC environment. This review of the prior art development of organophosphorus exchanger materials in Ca.sup.++ sensitive electrodes reveals a clear conception among those most skilled in the art, that an extreme high level of exchanger purity was not considered to be a critical factor in improving electrode operation. Instead, each successive increment of progress in enhancing sensitivity and sensitivity was attributed to variation in types of exchanger material, use of mediators, improved balance in relative concentrations of solid state constituents and engineering design of the electrode apparatus. The problems of response time and stability, however, continue to present difficulty in obtaining an operable Ca.sup.++ electrode system.